Radiation therapy is a highly effective modality to kill cancer cells, and we and others have demonstrated that both antigen adjuvant release following radiation can generate and enhance adaptive immune responses particularly when used with immunomodulatory therapies. However these effects are limited by tumor associated macrophages in the radiated tumor that direct tissue repair in response to interaction with irradiated cells that directly oppose anti-tumor immune responses. We propose targeting a specific pathway that directs this repair phenotype in macrophages following radiation using a novel phenotype. We hypothesize that the macrophage immune response to radiation can be redirected away from a repair phenotype and toward an adaptive promoting immune response using a novel Gas6-CD40L fusion protein. We propose in this study to perform initial characterization of this novel fusion protein. To do this we propose addressing the following specific aims. Aim1: Characterize the in vitro binding of a Gas6-CD40L fusion protein to dying cells and its ability to activate macrophages. Aim2: Define both the in vivo pharmacokinetic and toxicity profile of a Gas6-CD40L fusion protein in both tumor free and tumor bearing mice. Aim3: Test the in vivo function of a Gas6-CD40L fusion protein on tumor associated macrophages following focal radiation in tumor bearing mice. Our study design incorporates in vitro characterization of fusion protein binding and effect on macrophage polarization as well as in vivo studies of pharmacokinetics, toxicity and biodistribution. The successful completion of this project will provide the necessary preliminary data and feasibility for further development in more extensive studies.